A New Class of Large-amplitude Radial-mode Hot Subdwarf Pulsators

Using high-cadence observations from the Zwicky Transient Facility at low Galactic latitudes, we have discovered a new class of pulsating, hot compact stars. We have found four candidates, exhibiting blue colors (g - r ≤ -0.1 mag), pulsation amplitudes of >5%, and pulsation periods of 200-475 s. Fourier transforms of the light curves show only one dominant frequency. Phase-resolved spectroscopy for three objects reveals significant radial velocity, T _eff, and {log}(g) variations over the pulsation cycle, which are consistent with large-amplitude radial oscillations. The mean T _eff and {log}(g) for these stars are consistent with hot subdwarf B (sdB) effective temperatures and surface gravities. We calculate evolutionary tracks using MESA and adiabatic pulsations using GYRE for low-mass, helium-core pre-white dwarfs (pre-WDs) and low-mass helium-burning stars. Comparison of low-order radial oscillation mode periods with the observed pulsation periods show better agreement with the pre-WD models. Therefore, we suggest that these new pulsators and blue large-amplitude pulsators (BLAPs) could be members of the same class of pulsators, composed of young ≈0.25-0.35 M _⊙ helium-core pre-WDs.